Gas lasers and, in particular, helium-neon (HeNe) lasers are well known in the art. See, for example, the article by Bhogi Patel entitled "The Helium-Neon Laser: What It Is and How It Works" published in the January 1983 issue of Photonics Spectra, pp. 33-38. Typically, in such devices, a high d.c. voltage, in the range between 5KV and 10KV, is applied across the tube and its associated current-limiting series resistor in order to break down the gas and establish a steady-state discharge at a normal operating current of a few milliamperes. The process is set in motion when an initiating charged particle (most often an electron present within the gas volume between the tube electrodes, and subject to the electric field established by the applied voltage) gains sufficient energy to produce positive ion-electron pairs by collision with neutral gas atoms. The secondary electrons thus created are, in turn, accelerated by the electric field to produce additional ion-electron pairs, resulting in an exponential growth in the number of charged particles moving in the direction of the electric field. Breakdown, and a transition to the lower voltage, self-sustaining glow discharge mode, occurs when the positive ions formed move to the cathode and produce enough electrons by sedondary emission to replace the initiating electron.
The time delay between the application of the high voltage to the laser tube and the appearance of a glow discharge is termed the breakdown delay. This delay has two components. One component, the statistical delay, is associated with the time required for an ionizing particle to appear at an appropriate place in the tube to initiate breakdown. The other component, termed the formative delay, is the time required for the initial electron avalanche to build up to the point where a glow discharge appears. This latter delay, at the large voltages typically used to start gas lasers, is relatively small compared to the statistical delay and can be neglected for most practical applications. The statistical delay, on the other hand, can be significant.
The mechanical structure of modern gas laser tubes is determined largely by the need to maintain the laser mirrors, that define the laser cavity, in precise optical alignment despite changes in tube temperature and external mechanical vibrations and stresses. As will be described in greater detail hereinbelow, the tube typically includes; (a) a small bore capillary tube whose axis extends between the cavity mirrors, and within which discharge is made to occur; (b) a cylindrical cathode concentric with the bore and electrically connected to a cathode end cap that supports a first cavity mirror; (c) an outer enclosing glass cylinder; and (d) an anode end cap that provide an electrical connection and supports the second cavity-defining mirror.
In structures of this type, starting (i.e., statistical) delays as long as several seconds are often observed. The starting problem is most acute when the tube is shielded from external light sources, or is operated under conditions of very low temperature or humidity.
Techniques for reducing starting delay in gas laser are described, for example, in U.S. Pat. Nos. 3,792,372 and 4,190,810. The first of these patents discloses the use of an additional electrode or a wire loop connected to the power supply through a high resistance. In the second patent, a strip of electrically conductive plastic is disposed on the outer surface of the laser tube, one end of which is electrically connected to the laser anode. While such arrangements may reduce starting time, they do have a number of serious disadvantages. The first disadvantage is that they significantly increase the anode-to-ground capacitance, thereby greatly increasing the likelihood of producing parasitic relaxation oscillations. In order to minimize such oscillations, the added electrode is connected to the anode by means of a very large resistance. This, however, adds to the cost of the device, and presents obvious high-voltage insulation complications and safety hazards.
It is, accordingly, the broad object of the present invention to reduce the starting delay in cold cathode tubes, such at gas lasers.
It is a more specific object of the invention to reduce starting delay in gas lasers without significantly increasing cost or the likelihood of inducing parasitic oscillations.